1. Field of the Invention
This invention relates to Cr-Mo stainless steel used for making razor blades and showing a high resistance to corrosion, to razor blades, and also to a process for manufacturing razor blades.
2. Description of the Prior Art
High carbon steel containing 1.2% by weight of carbon and 0.4% by weight of chromium was usually used for making razor blades. This material showed a high degree of hardness when heat treated and could make a blade having a high level of cutting quality, but had the drawback of being poorly resistant to corrosion and of rusting easily.
Every razor is normally used in a more or less humid environment. When it is used, it is brought into contact with corrosive substances, such as the constituents of sweat, soap, and a shaving foam. Moreover, the nature of water which is used for shaving, and the temperature of the place where the razor is used, are likely to promote the rusting of its blade. The high carbon steel razor blade was primarily intended for providing a high level of cutting quality, and did not usually withstand any repeated use under the conditions as hereinabove stated.
Therefore, 13Cr martensitic stainless steel has come to be used widely as a rust-resisting material for making a razor blade having a high level of cutting quality. Martensitic stainless steel containing 0.6 to 0.7% of carbon and 12 to 13% of chromium, both by weight, is used more often for making razor blades than any other stainless steel. This material has a hardness of, say, HV 620 to 650 when heat treated, and is superior to high carbon steel in rusting and corrosion resistance owing to the 13% Cr which it contains.
This material is, however, not completely free from the problem of rusting, either; when it is used for making razor blades, it is usual practice to form a coating of e.g. platinum, chromium or chromium nitride (CrN) on the surface of the material by sputtering to improve its corrosion resistance. Although the coating does certainly improve the corrosion resistance of the material, a razor blade made of this material still has an undesirably short life due to the corrosion which occurs at the grain boundary, and the rust which forms between the coating and the substrate. Moreover, the formation of the coating requires additional equipment and incurs an additional cost.
DE-OS 1 533 380 discloses low carbon stainless steel as a razor blade material having corrosion resistance. This steel contains 0.32 to 0.44% of carbon, 11 to 16% of chromium, 0.2 to 0.5% of silicon and 0.2 to 0.5% of manganese, the balance of the composition being iron. It contains at least 75% of martensite and has a Vickers hardness (HV) of at least 500 (as tested under a load of 0.5 kg), if it is austenitized at a temperature between 1080.degree. C. and 1135.degree. C., hardened by cooling to a temperature between -25.degree. C. and -50.degree. C., and tempered. This material is intended for making a blade-forming band for a "band" razor. The band razor has a magazine for holding a band in the form of a roll from which the band can be unwound little by little to supply a part defining a new blade each time it has been unwound. Although this low carbon and high chromium steel may be satisfactorily resistant to corrosion and be sufficiently tough to be wound into a roll, its hardness as heat treated is too low to enable the manufacture of a blade having a high level of cutting quality.